External Beam Radiation Therapy

External beam radiation therapy is administered by directing a beam of photons at the tumor, using a linear accelerator (LINAC).

Photons have no mass and no electric charge. They behave both like elementary particles and like waves, forming part of the electromagnetic spectrum (high-energy x-rays).

How radiation worksThe radiation disrupts atoms and molecules in the cancer, forming ions (hence the term 'ionization').

Such ionizing radiation damages DNA so that cells can no longer replicate. As a result, they die, either immediately or when the time comes for them to divide, when they reach the end of their normal lifespan. For this reason, rapidly growing tumors with a short life-span (i.e., a short tumor doubling time) shrink more rapidly than those with a long tumor doubling time.

At high doses, this radiation also damages cell membranes and organelles causing immediate cellular disruption.

Side Effects of RadiationRadiation affects not only cancer cells but also normal tissues. This is called 'collateral damage'.

High does of radiation cause immediate effects, such as inflammation, redness, pain, swelling, ulceration and hair loss.

Lower doses of radiation do not immediately kill the cells but prevent them from replenishing themselves so that delayed effects occur. Examples of such side-effects include radiation retinopathy and radiation optic neuropathy. Much of these effects develop because the blood vessels become diseased (i.e., 'radiation vasculopathy), so that they leak and become obstructed, with the result that the tissues they nourish become edematous (i.e., waterlogged) and ischemic (i.e., starved of oxygen).

Radiation-vasculopathy within an irradiated tumor inside the eye can make the tumor toxic, because it leaks fluid into and under the retina and also because it produces hormones that stimulate the formation of abnormal blood vessels. These can cause hemorrhages in the eye, particularly vitreous hemorrhage, and can block the trabecular meshwork in the anterior chamber to cause painful neovascular glaucoma.

There are many other side effects. Some of these include cataract, dry eye if the lacrimal gland is affected, watery eye if the tear ducts are damaged.

Radiation damage to DNA can also cause new cancers to develop, particularly in patients who are predisposed to cancer, such as those with germinal retinoblastoma.

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